NDFB(S) insurgents threw a hand grenade at the Assam Rifles personnel but none was injured as it exploded in a water body near them.
This led to an exchange of fire between the two sides and an unidentified militant was killed while three others managed to escape.
Security forces later seized two Chinese made grenades and a pistol from the militant.
Search operations were launched to nab the remaining three members of the group, the sources added.